The present invention relates to a circuit which can be provided in a speakerphone, coupled, for example, in a telephone system, for enabling a central controling device, for example, a key service unit of a key telephone system, to determine whether a speakerphone is connected into the system. This is necessary so that the central processor in the key service unit can transmit the necessary commands to the speakerphone to control, for example, gain levels of the output signal from the microphone of the speakerphone or the input signals to the loudspeaker of the speakerphone.